1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, more particularly to a method for fabricating a low resistance titanium silicide film providing good heat-resistant properties.
2. Description of the Related Art
A conventional self-aligned- and silicidation method is described with reference to FIG. 12. First, a field oxide film 402, a gate oxide film 403 and a gate electrode 404 made of poly-crystalline silicon and having side walls covered with an insulating film 405 are formed on a semiconductor substrate 401 as shown in FIG. 12A. Then, as shown in FIG. 12B, after forming an oxide film 406, a high concentration of impurity ions are implanted through the oxide film 406 into the portions to be source/drain regions. Arsenic ions are implanted in the case of NMOS, while boron ions are implanted in the case of PMOS. Thereafter, annealing for activation is performed (e.g., in nitrogen gas ambient, at 900.degree. C. for 10 minutes) so as to form source/drain regions 407. Next, as shown in FIG. 12C, the oxide film 406 over the source/drain regions 407 end the gate electrode 404 is removed by using a solution containing hydrofluoric acid or the like, and thereafter a titanium metal film 408 is deposited by sputtering in argon gas ambient. Then, as shown in FIG. 12D, first RTA (Rapid Thermal Annealing) is performed for about 20 seconds at 650.degree. in nitrogen gas ambient, so as to make silicon and a titanium metal in the source/drain regions 407 and the gate electrode 404 react with each other, thereby forming titanium silicide films 409 each having a TiSi.sub.2 C 49 crystal structure which is stoichiometrically metastable (herein, the surface of the titanium metal film 408 changes into a titanium nitride film 410). Then, as shown in FIG. 12E, an unreacted titanium metal film 408 and the titanium nitride film 410 formed by the first RTA are removed by selective etching by using a mixed solution of sulfuric acid and a hydrogen peroxide solution. Thereafter, second RTA is performed at 800.degree. C. for about 20 seconds in nitrogen gas ambient so as to change the titanium silicide film 409 into a titanium silicide film 411 having a TiSi.sub.2 C 54 crystal structure which is, stoichiometrically stable.
In the method for fabricating a semiconductor device, after the step of forming transistors, an interlevel insulator is deposited on the transistor. Then, an annealing step for densifying and reflowing the interlevel insulator is performed. Normally, the annealing step is effective at 850.degree. C. or more, and preferably at 900.degree. or more.
However, the conventional process for forming a titanium silicide film has the following problems:
(1) The inclusion of oxygen resulting from process steps (e.g., a step of implanting impurity ions through the oxide film) cannot be prevented in the system of reaction of Ti and Si, no matter how the cleanliness of the instrument and surrounding conditions for the fabrication are improved. As a result, the silicidation reaction occurs in the ternary system of Ti, Si and O.
(2) In the silicidation reaction in the ternary system, SiO.sub.2 is predominantly formed in the grain boundary of TiSi.sub.2, which makes the sheet resistance higher and results in deterioration of heat resistance. This problem is prominent especially in silicidation for interconnection with a width less than the grain size of TiSi.sub.2. That is, in silicidation of the interconnection with a width less than the grain size of TiSi.sub.2, the change in crystal structure from C49 to C 54 is hard to occur in RTA at 900.degree. C. or less. As a result, a titanium silicide film having extremely high electrical resistance (sheet resistance) is obtained. In the case of performing the RTA at a temperature of 900.degree. C. or more, the change in crystal structure from C49 to C 54 is likely to occur. However, heat resistance deterioration and agglomeration of the silicide film are likely to occur in comparison with the silicide film obtained by silicidation of the interconnection with a large width. In addition, since the agglomeration of the silicide film starts also in the silicide film with a further large width, the agglomeration is inevitable.
(4) In the case where the silicide film is formed by the conventional silicidation method and then annealing at a temperature of more than 800.degree. C. is performed for the reflow of the interlevel insulator, the agglomeration occurs and the junction leakage of the source and drain regions increases, whereas the reliability of the gate oxide film deteriorates. In addition, the interconnection resistance rises, and especially, in the case of the silicidation of the interconnection (e.g., a gate electrode) with a width smaller than the grain size of TiSi.sub.2, the sheet resistance rises to the level as high as the level in the interconnection not having a silicide film.